In Ehrlich ascites carcinoma cells, acidification of the cytoplasm down to pH 6.2-6.3 arrests DNA synthesis. Such acidification can be obtained by decreasing the pH outside the cell or, alternatively, by addition of a micromolar concentration of the K+/H+ antiporter nigericin. Thus, nigericin may be regarded as a new type of cytostatic, the effect of which is mediated by alteration of the intracellular pH.